1. Field of the Invention
The present invention relates generally to a bone screw for drawing together portions of a bone separated by a fracture and more particularly to such a screw which draws the bone portions together as a result of different-pitched threads on the screw.
2. Description of the Related Art
In healing bone fractures it is desirable to compress the fractures so that the fractured surfaces are pressed against one another. In the prior, art bone screws have been used to draw the fractured surfaces together and thereby optimize the healing process.
A number of prior art bone screws are constructed in similar to wood screws. For example, some prior art bone screws include a threaded distal portion and a head having a relatively long unthreaded shank disposed between the head and the distal portion. A drill is used to bore a hole through the fracture and the screw is threaded into the remote bone fragment with the head of the screw compressing the near fragment tightly against the remote bone fragment.
Other bone screws are threaded along the length thereof thus requiring a first drill bit to bore a hole in both bone fragments across the fracture and a second bit to drill a larger hole in the near bone fragment so that the screw threads do not engage the near bone fragment. Thereafter, the screw is tightened in the same manner as described above in connection with the screw having an unthreaded shank thereby compressing the fragments together.
Another bone screw is described in U.S. Pat. No. 4,175,555 to Herbert. The Herbert bone screw includes a shaft having leading and trailing portions with a first screw thread formed on the leading portion. A second screw thread is formed on the trailing portion which is like-handed but of a smaller pitch than the first screw thread. A slot or hex socket is formed on the trailing portion to accommodate a driver for driving the screw into a bore formed across a bone fracture.
As noted in the Herbert patent, bone screws having heads suffer from several disadvantages including concentrated loads beneath the screw head and the provision of the screw head itself after the screw is installed. Several other shortcomings of the standard type of bone screw are detailed in the Herbert patent.
The Herbert bone screw is advantageous in that it eliminates a conventional screw head on the trailing end portion of the screw. The Herbert bone screw, however, itself suffers from a number of disadvantages.
In the Herbert screw, the leading threads have a smaller diameter than the trailing threads. This is necessary to permit the leading threads to pass through the relatively large bore in the near bone fragment and engage the smaller bore in the remote bone fragment. The larger trailing threads then engage the larger bore in the near bone fragment. As a result of this arrangement, any stripping of the threads cut into the bones during installation of the screw occurs in the remote bone. If the stripping occurred in the bore in the near bone fragment, a screw having a head thereon could still be used to compress the fracture even though the near bore was stripped; however, when stripping occurs in the bore in the remote bone, a standard screw with the head thereon cannot be used and another bore must be drilled.
Further, the Herbert screw must be correctly positioned, i.e., it is imperative that the fracture intersect the unthreaded central portion of the Herbert bone screw when the same is installed. In addition, because the Herbert screw is not threaded entirely along the length thereof, the purchase obtained by the screw in the bone is not as good as with a screw threaded along the entire length. Also, two bores of different sizes must be drilled to install the Herbert screw rather than a single bore.
It would be desirable to provide a headless bone screw which overcomes the disadvantages associated with the Herbert bone screw.